In recent years, various line interface circuits have been developed wherein the tip and ring leads of a subscriber's loop are terminated directly or indirectly at tip and ring active feed means, as exemplified in each of the following listed U.S. Pat. Nos.:
4,321,430--Ferrieu (Mar. 23, 1982) PA0 4,387,273--Chea, Jr. (June 7, 1983) PA0 4,484,032--Rosenbaum (Nov. 20, 1984) PA0 4,514,595--Rosenbaum et al (Apr. 30, 1985) PA0 4,539,438--Rosenbaum et al (Sept. 3, 1985) PA0 4,571,460--Rosenbaum et al (Feb. 18, 1986)
In the later four listed patents, examples of line circuits described therein usually include a.c. and d.c. feedback networks or the like which serve to determine the effective operating output impedances of the active feed means.
It is an object of the invention to provide a line interface circuit wherein a.c. line impedance termination and d.c. line feed resistance termination are segregated and independent one from the other.
In each of the four examples, a non-linear element may be combined with the d.c. feedback network whereby line feed current is limited to a predetermined value by increasing the resistances of the active feed means. This feature is useful for conservation of current supply on short subscriber loops, however it may be deleterious to voice quality as it effectively inhibits the equalization characteristics of a typical telephone set remotely connected to the short communication line. In the designs of typical telephone sets, response characteristics of both the transmitting and receiving apparatus therein have been taylored to compensate for the lesser signal loss on short loops. The transmitting and receiving apparatus are arranged to be progressively less sensitive in the presence of line current in excess of about 40 milliamps. Therefore, in short loops a function of line current limiting increases a.c. signal levels, received from the typical telephone, to beyond that normally expected in the telephone system.
Another problem in termination of telephone lines is that the typical line interface circuit is adaptable to one of only two specified standard impedances. One being specified for the majority of telephone lines and the other being specified for extremely long telephone lines which are inductively loaded to enhance analog voice band transmission. In actual practice however, not all telephone lines are of one or the other standard impedance, and hence in full duplex operation the hybrid circuit function is less than optimal and may permit oscillation sometimes referred to as singing because of insufficient return loss. Operating telephone companies usually avoid this problem by a simple expedient of having about eight decibels of loss in each line interface circuit. Hence in a typical telephone conversation between telephone subscribers, about sixteen decibels of loss is inserted between the transmitter of one subscriber and the receiver of another subscriber. Recently, some telephony networking features of private branch exchanges in combination with one or more operating companies involve four or more two-wire communication lines and line interface circuits in a tandem connection. In such an arrangement, typically more than forty decibels of loss is inserted, making conversation difficult in all but the quietest of environments.
It is an object of the invention to obtain an effective measure of the communication line from a current limiting action in the d.c. line feed resistance termination, and to use this measure to vary the a.c. line impedance termination accordingly whereby attenuation and return loss values, in a group of typical telephone lines, are more consistently optimized.
One structural characteristic common to many of the examples in the previously mentioned patents is that of a resistance network which includes tip and ring feed resistors arranged in series between tip and ring leads of the communication line and tip and ring active impedance feed means. The resistance network also includes tip and ring voltages dividers including tip and ring taps from whence d.c. voltages are utilized for detection of supervisory states and a.c. voltages are utilized to receive voice band information and to dynamically control the tip and ring active impedance feed means. As a practical matter, it is preferred that the resistance network be provided on a supporting substrate and that individual resistive elements be trimable to achieve close ohmic matches between various of the resistors. If the line interface circuit is to be adapted for higher than voice band frequency signals, as for example may be the case of terminating a "U" interface in an integrated subscriber digital network (ISDN), parasitic capacitances associated with the physical resistance elements must also be closely matched. Any reactive mismatch, for example a picofarad or so, is amplified by following amplifier elements, such as to render the line circuit virtually useless at ISDN operating frequencies. Unfortunately, precision matching of the parasitic capacitance to the degree required is at least for the present impractical.
It is therefore an object of the invention to provide a line interface circuit having a practical operating band width of about 200 khz, which includes the audio spectrum and the ISDN 2B+D basic rate service U interface band width requirements.